Oxygen is an economically important gas which is widely used in large-scale industrial applications. New uses for oxygen are emerging in advanced high-temperature processes for iron and steel manufacture, coal gasification, oxygen-enriched combustion, and in particular integrated gasification combined cycle power generation. In these large-scale applications, the cost of oxygen produced by conventional cryogenic or noncryogenic technology is a major portion of the overall operating cost, and lower oxygen cost will encourage the commercialization of these emerging technologies. New oxygen separation processes which can be themally integrated with these advanced high-temperature processes will reduce the energy consumed in oxygen production, which in turn will promote the technical and commercial development of such integrated systems.
Oxygen can be recovered from air at high temperatures by inorganic oxide ceramic materials utilized in the form of selectively permeable non-porous ion transport membranes. An oxygen partial pressure differential or a voltage differential across the membrane causes oxygen ions to migrate through the membrane from the feed side to the permeate side where the ions recombine to form electrons and oxygen gas. An ion transport membrane of the pressure-driven type is defined herein as a mixed conductor membrane, in which the electrons simultaneously migrate through the membrane to preserve internal electrical neutrality. An ion transport membrane of the electrically-driven type is defined herein as a solid electrolyte membrane in which the electrons flow from the permeate side to the feed side of the membrane in an external circuit driven by a voltage differential. A comprehensive review of the characteristics and applications of such membranes is given in a report entitled "Advanced Oxygen Separation Membranes" by J. D. Wright and R. J. Copeland, Report No. TDA-GRI-90/0303 prepared for the Gas Research Institute, September 1990.
In the recovery of oxygen from air at high temperatures (typically 700.degree. C. to 1100.degree. C.) using ion transport membranes, a significant amount of heat energy is available in the membrane permeate and non-permeate streams. The effective use of this energy in the overall operation of a mixed conductor membrane system is necessary if the system is to be competitive with conventional cryogenic technology for large scale oxygen production. Energy recovery and effective utilization thereof is possible by the integration of compressors, combustors, hot gas turbines, steam turbines, and heat exchangers with the mixed conductor membrane module. U.S. Pat. No. 4,545,787 discloses the production of oxygen and net power in the integrated operation of a mixed conductor ceramic membrane. Air is compressed, heated, and passed through a membrane separator to produce an oxygen permeate and an oxygen-containing non-permeate stream. The non-permeate stream is combusted with a fuel and the hot combustion gases are expanded in a hot gas turbine. The turbine provides shaft power for the compressor and drives a generator for export of electricity, and turbine exhaust is optionally used to cogenerate steam or to preheat the compressed air membrane feed. Alternately, the membrane is placed downstream of the combustion step.
U.S. Pat. No. 5,035,727 describes the recovery of oxygen by a solid electrolyte membrane in conjunction with an externally-fired gas turbine in which compressed air is heated indirectly and passed through the membrane module. Non-permeate gas is expanded through a hot gas turbine, the turbine exhaust is heated by direct combustion, and the combustion products provide heat indirectly to the membrane feed. Steam is recovered from the waste heat after heat exchange with the membrane feed.
U.S. Pat. No. 5,118,395 describes the recovery of oxygen from gas turbine exhaust utilizing a solid electrolyte membrane with the coproduction of electric power and steam. A supplemental burner heats the turbine exhaust prior to the membrane, and steam is generated by the membrane non-permeate stream. Related U.S. Pat. No. 5,174,866 discloses a similar system in which intermediate turbine exhaust is passed through the membrane and the membrane non-permeate stream is further expanded through another turbine stage. In both patents, turbine shaft power is used to drive the air compressor and an electric generator.
The report by J. D. Wright and R. J. Copeland identified above discloses at p. 55 a gas turbine-driven ceramic membrane system in which air is compressed, heated indirectly in a fired heater, and passed through the membrane to yield oxygen and non-permeate gas. The non-permeate gas is combusted with natural gas in the fired heater and the combustion products are expanded through a hot gas turbine to drive the compressor and generate electric power. Heating of the air feed to the membrane and the combustion of fuel and non-permeate gas prior to the turbine thus are accomplished in a single integrated combustion chamber.
U.S. Pat. No. 5,245,110 (equivalent to PCT International Publication No. WO 93/06041) discloses the integration of a gas turbine with an oxygen-selective membrane system. The permeate side of the membrane is swept with air to yield an enriched air product containing about 35 vol % oxygen. The enriched air product is used in a hydrocarbon reformer or gasifier process, and tail gas from the reformer or gasifier is introduced into the gas turbine combustor to balance the flow of hot gas to the turbine. The nitrogen from the permeate and membrane sweep air replaces the mass lost when oxygen is consumed in the reformer or gasifier process, which maintains the turbine in a desired mass and thermal balance.
An article entitled "Separation of Oxygen by Using Zirconia Solid Electrolyte Membranes" by D. J. Clark et al in Gas Separation and Purification 1992, Vol. 6, No. 4, pp. 201-205 discloses an integrated coal gasification-gas turbine cogeneration system with recovery of oxygen for use in the gasifier. Membrane non-permeate is combusted with gas from the gasifier and passed to the gas turbine cogeneration system.
A combined cycle power generation system is a highly efficient system which utilizes a gas turbine to drive an electric generator, wherein heat is recovered from the turbine exhaust as steam which drives an additional electric generator. A description of typical combined cycle power generation systems is given in The Chemical Engineer, 28 Jan. 1993, pp. 17-20. The compressor, combustor, and expansion turbine are carefully designed and integrated to maximize the efficiency of each component and thus the efficiency of the integrated system. Preferably these systems are operated at steady-state design conditions, since significant deviations from these conditions will adversely affect system efficiency.
The successful development and commercialization of oxygen production by ion transport membranes will require flexible systems which maximize energy utilization and allow operation of system components at optimum conditions. In many situations, the recovery of oxygen and excess energy from an ion transport membrane system must be maximized in order to minimize operating costs. In other situations, low capital cost will be of primary importance and operating cost of less importance. The invention disclosed below and described in the following claims addresses these latter situations with a method for producing oxygen by an ion transport membrane system in which water is introduced into the system to improve oxygen recovery and heat utilization.